Exploration and production of hydrocarbons from subsea reservoirs is an expensive and time-consuming process. The drilling and production processes often require allocation of expensive assets, such as drilling and production platforms located offshore. There are a number of problems associated with offshore drilling and production that not found in land operations.
Primary among these is the marine environment. Drilling equipment is subject to ocean currents. The currents apply forces on subsea elements, subjecting them to additional loading and stress. The currents also can present a problem with maintaining the drilling platform at a desired location. Moreover, unlike the surface environment, offshore drilling control equipment is generally located on the seabed and not subject to direct control and monitoringxe2x80x94one simply cannot see the equipment without the use of vision equipped ROVs. The marine environment also presents a biological challenge in that marine organisms may congregate and grow on subsea equipment, possibly interfering with operations. Yet another problem is the chemistry of the marine environment. Salt in the water creates an environment that can promote cathodic attack on subsea equipment. This environment can affect all marine elements from moorings, tendons, tension leg platforms, as well as drilling and production risers.
The mechanics of drilling in a marine environment also differ from land operations. Drilling operations utilize a drilling fluid, known as xe2x80x9cmudxe2x80x9d which is pumped down the drill string and circulated back to the surface through an annulus between the drill string and the borehole wall. The drilling mud cools the drill bit as it rotates and cuts into the earth formation. It also provides a medium for returning drilling cuttings created by the drill it to the earth""s surface via the annulus. The weight of the drilling mud in the annulus further operates to control pressure in the borehole and help prevent blowouts. Lastly, additives in the mud are designed to form a cake on the inside walls of the borehole to provide borehole stability and to prevent formation fluids from entering the borehole prior to desired production. It will be appreciated that during land operations, the drilling mud and cuttings may be readily returned to the surface via the borehole annulus. Such is not the case in offshore operations.
Offshore operations require location of a drilling platform in waters located generally above the reservoir of interest. The depth of the water may range from several hundred feet to almost half a mile. A drill string must travel from the surface of the platform, down to the seabed and then into the formation of interest prior to actually beginning cutting operations. Unlike land operations, there is no annulus between the floor of the seabed and the drilling platform at the surface. Accordingly, a drilling xe2x80x9criserxe2x80x9d comprised of generally cylindrical elements is provided for from below the seabed to the surface drilling platform above the water level. The riser operates to protect the drilling string during operations and acts as an artificial annulus.
The risers are formed from large (on the order of 21 inches) diameter metal tubular goods linked together. Buoyancy elements, often manufactured from synctatic foam or metal, may be affixed the external surface of the drilling riser along its length to provide essentially neutral buoyancy. The synctatic foam buoyancy elements are typically 6-12 feet in length. The specific foam chemistry and diameter of the float are selected in accordance with the specific environmental conditions to be encountered in operations. Riser joints may be as long as 75 feet or more in length and multiple buoyancy-elements may be affixed to a single riser joint. The buoyancy elements are generally manufactured onshore and shipped, together with the riser joints, to the drilling platform prior to use. The buoyancy elements are usually installed on the riser prior to riser installation. The foam floats may be affixed about the riser elements any number of ways as will be discussed with reference to the preferred embodiments of the invention.
Often, the riser and other subsea elements, including buoyancy elements, is subjected to ocean currents along its length, causing lateral deflection in the riser from the seabed to the surface platform. A riser may be subjected to varying and differential ocean currents along its length resulting in complex lateral deflection of the riser. This results in a number of problems. The continued deflection of the riser may result is stress points along its length and ultimately weaken the riser. Radical lateral deflection in the riser could result in excessive drill string contact with the inside riser wall resulting in further weakening of the riser.
The currents have yet another effect in that they created a drag force on the riser, causing vortexes to shed from the sides of the elements as the currents move around the generally cylindrical elements. These vortexes can cause further increased deflection of the riser. Moreover, they can make it more difficult to keep the drilling platform positioned. These forces may result in excessive drilling angles as seen from the platform, limiting the nature of drilling operations. The vortexes often create a Vortex Induced Vibration (VIV) effect in the riser resulting in further complex dynamic movement in the riser system, which could result in failure of riser structural elements. The drag on a cylindrical body submerged in a moving fluid is related to the Reynolds number for the body, where the Reynolds number is defined as xcfx81vd/xcexc, where xcfx81 is the fluid density, v is the fluid velocity, d is a characteristic length, and xcexc is the fluid dynamic viscosity.
A number of different solutions to deal with the problem of VIV, including the use of wing-like fairings enclosing the buoyancy elements to reduce vortex effects as disclosed in U.S. Pat. No. 6,048,136, assigned to the assignee of the present invention. However, there are a number of problems associated with the use of fairings. They typically do not fit through the rotary table of a drilling rig and must be added below the table. Further, there are additional storage and handling problems associated with the use of fairings.
Yet another approach is the use of smooth cylindrical riser elements having Reynolds numbers in excess of 100,000, as disclosed in U.S. Provisional Application No. 60/154,289, filed Sep. 16, 1999, likewise assigned to assignee of the present invention.
The present invention is directed to an apparatus and method to reduce drag and resulting vortex induced vibrational loading and stress in drilling and production risers, moorings and other subsea elements through the use of buoyancy elements having an ultra-smooth surface applied to the buoyancy element.